📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/cross-spawn/lib/util/readShebang.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const fs = require('fs');
+const shebangCommand = require('shebang-command');
+
+function readShebang(command) {
+    // Read the first 150 bytes from the file
+    const size = 150;
+    const buffer = Buffer.alloc(size);
+
+    let fd;
+
+    try {
+        fd = fs.openSync(command, 'r');
+        fs.readSync(fd, buffer, 0, size, 0);
+        fs.closeSync(fd);
+    } catch (e) { /* Empty */ }
+
+    // Attempt to extract shebang (null is returned if not a shebang)
+    return shebangCommand(buffer.toString());
+}
+
+module.exports = readShebang;